For the force of Earth on the Moon, \(m_2\) is the Moon’s mass and \(r\) is the Earth-Moon distance. If Equation 10-1 is true for any two objects, then it must also be true that the object of mass \(m_2\) exerts a gravitational force on the object of mass \(m_1\), and this force is directly proportional to the mass \(m_1\). But Newton’s third law (Section 4-5) tells us that the forces that the two objects exert on each \perpher have opposite directions and the same magnitude: \(F_{\mathrm{2\ on\ 1}} = F_{\mathrm{1\ on\ 2}}\). Hence the gravitational force that each object exerts on the \perpher must be directly proportional to both \(m_1\) and \(m_2\). This chain of reasoning leads us to Newton’s law of universal gravitation: